It is known that a magnetic separator is designed to extract from a flow of mixed materials all those parts having magnetic permeability, so as to separate them from the rest of the inert material. A typical separator essentially consists of a magnetic pulley, acting as driving roller, which draws a belt that conveys a mix of materials, the belt being closed in a loop around a return roller.
Magnetic pulleys with different magnetic field gradient suitable to separate materials with high or low magnetic permeability are used to select the material. With a low field gradient only materials with high magnetic permeability are attracted, whereas with a high field gradient both high magnetic permeability and low magnetic permeability materials are attracted.
A drawback of known separators, in particular those with high field gradient pulley, is that the material attracted by the corresponding polarities remains attached to those polarities until the conveyor belt moves away from the roller thus causing the detachment of the attracted material in a very small area. As a consequence, both low magnetic permeability and high magnetic permeability materials fall in the same area and have to be subsequently sorted.
Another drawback stems from the fact that the magnetic materials bring along a portion of the inert material, since the latter remains pinched between the inductor (the alternate polarities of the roller) and the induced (the attracted magnetic material). Therefore also in this case a further working is required to increase the quality of the selected material.
Another type of magnetic separator is the eddy current separator that is used to separate non-magnetic yet electrically conductive materials such as aluminum, copper, brass, etc. In this case there is provided a magnetic roller that rotates at high speed inside a non-magnetic tube around which the conveyor belt is wound.
The rotational speed of the roller must be very high (e.g. 3000 rpm) to induce in the conductive materials the eddy currents that in turn due to the fast variation of the magnetic field cause a repulsion of said materials that are thus separated from the mix. Moreover, in order to achieve the maximum operational efficiency the gap between the magnetic roller and the non-magnetic tube must be as small as possible, and this can cause overheating problems due to the high relative rotational speed between the two members. An example of such a separator for conductive materials is found in U.S. Pat. No. 5,394,991.